The three–loop massless Wilson coefficients are expressed by effective representations. Otherwise we use the analytic
Mellin space representations. After algebraic reduction [1]},
they depend on 32 harmonic sums [2]}, [3]} up to weight
w=6 only, which is particular to the flavor non–singlet case
to three–loop orders. Other massive Wilson coefficients have a more involved structure
[4]}, [5]}, [6]}, [7]}. It is useful to represent
harmonic sums
with an alternating index by their Mellin transform, to eliminate spurious \((-1)^N\)  terms [3]} before
performing the analytic continuation from even or odd integers to the complex plane. The singularities of the problem
are located at the non–positive integers, around which the contour integral is performed,
cf. [9]}, [10]}, [11]}, [12]}. For the analytic continuation we follow
Refs. [13]}, [14]}.
